1. Field of the Invention
The present invention relates to a manufacturing method for an image pickup apparatus, and more particularly to a producing method for a MOS image pickup apparatus.
2. Related Background Art
As a photoelectric converting device for use in a digital still camera or a digital video camera, a CMOS area sensor is often employed. In a CMOS area sensor, electrical signals generated by photoelectric conversion by a plurality of photodiodes provided on a semiconductor substrate are read by MOS transistors. It has, for example, a following structure as disclosed in Japanese Patent Application Laid-Open No. 2002-231915. On an N-type silicon semiconductor substrate, a well is formed by diffusing a P-impurity, and an N-impurity diffusion area of a photodiode is formed in such P-well. A charge generated by a photoelectric conversion is accumulated in the N-impurity diffusion area. Subsequently, the charge is transferred, by a gate electrode of a charge-transferring MOS transistor, to a drain area (floating diffusion area), constituted of an N-impurity diffusion area, of the transfer MOS transistor. Such photodiode, gate electrode and drain area are covered by a first insulation film. A first wiring layer is provided on the first insulation film and is connected with an impurity diffusion area such as the drain area of the transfer MOS transistor, through a contact plug penetrating the first insulation film. On the first wiring layer, there are similarly provided a second insulation film, a second wiring layer, a third insulation film and a third wiring layer. The first and second wiring layers are connected by a via plug penetrating the second insulation film, and the second and third wiping layers are connected by a via plug penetrating the third insulation film. The gate electrode is also connected by a similar wiring structure. Such contact plug and via plug are often formed by tungsten. On the third insulation film, a passivation film for device protection is provided, and a color filter and a microlens for sensitivity improvement are provided on the passivation film.
The CMOS area sensor of such structure is prepared by at first forming a lowermost part such as the photodiode, then forming a first insulation film, and forming a contact plug penetrating the first insulation film. The process is following by forming a first wiring layer, then forming a second insulation film in a similar manner, and forming a via plug penetrating the second insulation film. It is then followed by forming a second wiring layer, forming a third insulation film in a similar manner, forming a via plug penetrating the third insulation film and forming a third wiring layer.
For forming a contact plug or a via plug, a connecting hole is formed in the insulation film by a photolithographic process and an etching process. Then a high-melting metal or alloy film such as of tungsten is formed on the insulation film by a blanket chemical vapor deposition (CVD). The blanket CVD process causes a metal film deposition on a surface bearing the connecting hole, and fills the interior of the connecting hole with a metal film. Thereafter the metal film is removed except for the connecting hole to obtain a contact plug or a via plug. At the same time the insulation film is planarized.
For removing the metal film except in the interior of the connecting hole and planarizing the insulation film, following methods are known. Japanese Patent Application Laid-Open No. 2003-204055 discloses an etch-back method for planarizing irregularities of a film by an etching, and Japanese Patent Application Laid-Open No. 2001-352049 discloses a chemical-mechanical polishing (CMP). In the prior technology, such etch-back method or CMP method has always been selected for removing the metal film on the insulation film.
The etch-back method, when employed for forming a contact plug or a via plug, allows to attain a uniform flatness on the insulation film, but is unable to completely remove a residue of the metal film, caused in the etching process, thereby causing a possibility of shortcircuiting on the wiring layer to be formed thereon.
On the other hand, the CMP method, when employed for forming a contact plug or a via plug, can remove protruding extraneous matters deposited in the steps up to the metal film formation and can reduce the possibility of shortcircuiting of the wirings in comparison with the case of the etch-back method. However, since the insulation film is also polished together with the contact plug or the via plug, a polishing unevenness is caused depending on a CMP apparatus and a polishing pattern.
Because of such polishing unevenness, the insulation to be positioned between the photodiode and the color filter becomes uneven in the thickness for example between a central part and an edge part of a sensor chip. Particularly in an image pickup device, the incident light amount entering the photodiodes becomes uneven, thereby leading to a deterioration in the sensor characteristics, such as an unevenness in color. Particularly a thickness unevenness in the uppermost insulation, in the laminating direction, may influence significantly the sensor characteristics.